Lot of times I have to create a Dictionary<KeyType, List<ValueType>>
Before I can start using the dictionary I have to first verify that List has been created for that key.
//Can i remove these two lines?
if(!dict.ContainsKey(key))
dict[key]= new List<ValueType>;
//now use the key
dict[key].Add(value);
I know its only "2 lines" of code but it annoys me and I think it can be removed.
I can extend dictionary in someway but before I do it, I want to know if someone has found a clever way to remove the above if statement.
Basically i want to create a Dictionary<KeyType, Collection<ValueType>> and start using it right away like dict[key].Add(value).
You could create something like Google Java Collection's Multimap... or you could just add an extension method like this:
public static void AddValue<TKey, TValue>
(this IDictionary<TKey, List<TValue>> dictionary, TKey key, TValue value)
{
List<TValue> values;
if (!dictionary.TryGetValue(key, out values))
{
values = new List<TValue>();
dictionary.Add(key, values);
}
values.Add(value);
}
As Bevan says, Lookup can help as well - but you can only create one with the ToLookup method, and you can't modify it afterwards. In many cases that's a thoroughly good thing, but if you need a mutable map then you'll ned something like the above.
Have a look at the LookUp class introduced with Linq in .NET 3.5 - it might be just what you're looking for: a Dictionary like class that supports multiple items per key.
Perhaps the only significant downside is that you have to have all your elements available in one batch, as LookUp is immutable.
The ConcurrentDictionary<T,K>.GetOrAdd method is amazingly useful.
private ConcurrentDictionary<string, ICollection<int>> _dictionary;
private static ICollection<int> CreateEmptyList(string dummyKey)
{
return new List<int>();
}
private void AddValue(string key, int value)
{
ICollection<int> values = _dictionary.GetOrAdd(key, CreateEmptyList);
values.Add(value);
}
Edit: Here's an example of how to implement the feature as an extension method for IDictionary<T,K> (C# 3):
Note that IDictionary<TKey, TValue> is generally not thread safe, so if you want thread safety with this extension method you'll have to manually implement it just as for other operations.
public static TValue GetOrAdd<TKey, TValue>(
this IDictionary<TKey, TValue> dictionary,
TKey key,
Func<TKey, TValue> valueFactory)
{
TValue value;
if (!dictionary.TryGetValue(key, out value))
{
value = valueFactory(key);
dictionary.Add(key, value);
}
return value;
}
To add to the answers, you can also add a more general extension which accepts a delegate for instantiation:
public static TValue GetOrCreate<TKey, TValue>
(this IDictionary<TKey, TValue> dict,
TKey key,
Func<TKey, TValue> getValue)
{
TValue value;
if (!dict.TryGetValue(key, out value))
{
dict.Add(key, getValue(key));
}
return value;
}
and then you can provide whatever instantiation method you like:
Dictionary<int, string> dict = new Dictionary<int, string>();
string result = dict.GetOrCreate(5, i => i.ToString());
Related
Which is the difference between doing this:
public static bool IsNullOrEmpty<TKey, TValue>
(this Dictionary<TKey, List<TValue>> dictionary, TKey key)
{
return !dictionary.ContainsKey(key) ||
dictionary.ContainsKey(key) && dictionary[key].Count == 0;
}
And this:
public static bool IsNullOrEmpty<TKey, TValue>
(this Dictionary<TKey, TValue> dictionary, TKey key)
where TValue : List<TValue>
{
return !dictionary.ContainsKey(key) ||
dictionary.ContainsKey(key) && dictionary[key].Count == 0;
}
As far as I can notice, the compiler doesn't tell me that there isn't anything wrong. But, is this any approach better than the other? Will it have any different return value (because I don't realize about this yet)?
First off: your best bet for answering this question yourself would have been try it. You would have soon seen that when you attempted to call the function in its second form, it does not work.
That said, let's dig into this a bit. We have
public static bool IsNullOrEmpty<TKey, TValue>(
this Dictionary<TKey, List<TValue>> dictionary, TKey key)
versus
public static bool IsNullOrEmpty<TKey, TValue>(
this Dictionary<TKey, TValue> dictionary, TKey key)
where TValue : List<TValue>
Why is the second one wrong? Well, what type argument would you like to pass for TValue? Suppose we have a Dictionary<string, List<int>> in hand. What is the TValue we can use? It's not int, because that doesn't meet the constraint: int does not derive from List<int>. But it is not List<int> either because List<int>> does not derive from List<TValue>, which is List<List<int>>.
So, now we know why the second one is wrong. Let's now answer more questions:
Under what circumstances does this sort of "recursive" constraint make sense?
Suppose we are trying to find the maximum key in a dictionary:
public static TKey MaxKey<TKey, TValue>(
this Dictionary<TKey, TValue> dictionary)
where TKey : IComparable<TKey>
{
if (dictionary.Count == 0) throw ...
TKey best = default(TKey);
bool first = true;
foreach(TKey k in dictionary.Keys)
{
if (first || best.CompareTo(k) < 0)
best = k;
first = false;
}
return best;
}
Here it totally makes sense to constrain TKey to IComparable<TKey>; we're going to compare keys.
What are some other ways that people use and misuse this pattern?
See https://blogs.msdn.microsoft.com/ericlippert/2011/02/03/curiouser-and-curiouser/ for many examples.
Why is the first method less than perfect?
Because it does not handle these two cases:
First, suppose we have a multidictionary but it is not from keys to lists:
Dictionary<string, int[]>
or
Dictionary<string, Stack<int>>
or
Dictionary<string, IEnumerable<int>>
or whatever.
Second, it also does not handle the case
class MyList : List<int> {}
...
Dictionary<string, MyList>
though that case is pretty rare; you ought not to be extending List<T> normally.
What is the right way to implement my dictionary method for maximum generality?
Here's one way:
public static bool IsEmpty(this IEnumerable items)
{
// EXERCISE: Why is this implementation bad?
// EXERCISE: Can you improve it?
foreach(var item in items)
return false;
return true;
}
public static bool IsNullOrEmpty<TKey, TValue>(
this Dictionary<TKey, TValue> dictionary, TKey key)
where TValue : IEnumerable
{
return !dictionary.ContainsKey(key) || dictionary[key].IsEmpty();
}
For even more generality you might use IDictionary instead of Dictionary.
Wondering whether it is possible to have a method with a dictionary parameter where TKey and TValue types are indeterminate. For example, I may want to send a key of int and a value as a custom object, or a key of string and value of a different object.
Can this be done via generic objects, is the best practice with overloads, or is there another method I have not yet located?
public static SortedDictionary<??, ??> CreateSortedDictionary(Dictionary<??, ??> d)
{
SortedDictionary<??, ??> rv = new SortedDictionary<??, ??>();
enter code here...
return rv;
}
Thank you.
You can use generic types for the key and value, which can then be used for both the input and output dictionaries:
public static SortedDictionary<TKey, TValue> CreateSortedDictionary<TKey, TValue>(
Dictionary<TKey, TValue> input)
{
return new SortedDictionary<TKey, TValue>(input);
}
Many times over the years, I have needed code that does:
Find a value in a dictionary; if it is not there, add it to the dictionary (and return that new value).
For example:
// Only one per account, so loading can be efficiently managed.
// <AccountID, LCProfilePicture>
public readonly static Dictionary<int, LCProfilePicture> All = new Dictionary<int, LCProfilePicture>();
public static LCProfilePicture GetOrCreate( int accountID )
{
LCProfilePicture pic;
if (!All.TryGetValue( accountID, out pic )) {
pic = new LCProfilePicture( accountID );
All[ accountID ] = pic;
}
return pic;
}
Instead of having to write that boilerplate each time, I'd like to have a generic method that will do the work. How to do so in c#?
So far, I have thought of three ways to proceed:
Wrap the construction that will be needed if the dictionary does not already contain an object for the key, into an Action (or Func?). Then call that if necessary.
Require TValue to have a constructor of that form, and then somehow describe that requirement as a constraint on the generic method.
Define some interface that TValue has to satisfy, and somehow use that interface to write the generic method.
I think I know how to do #1, so will submit an answer doing so, as soon as I work out the details. UPDATE: I have now worked that out, and posted that as an answer.
But maybe #2 is possible? Then I could just add that constraint, and be done.
Pro: easier to use (don't have to wrap the construction into an Action or Func).
Con: Not as flexible (if have a TValue that does not have such a constructor, can't use this generic method).
(#3 seems less promising; I mention it for completeness.)
You can combine constraints of new() and an interface for setting the key, like this:
interface IWithKey<T> {
public T Key { get; set; }
}
static class DictExtensions {
public static TVal GetorCreate<TKey,TVal>(this IDictionary<TKey,TVal> d, TKey key) where TVal : new(), IWithKey<TKey> {
TVal res;
if (!d.TryGetValue(key, out res)) {
res = new TVal();
res.Key = key;
d.Add(key, res);
}
return res;
}
}
Since GetorCreate is an extension, you can use it as follows:
static LCProfilePicture GetOrCreatePic( int accountID ) {
return All.GetOrCreateEntry(accountID);
}
I noticed in your example you have a static dictionary
// Only one per account, so loading can be efficiently managed.
// <AccountID, LCProfilePicture>
public readonly static Dictionary<int, LCProfilePicture> All =
new Dictionary<int, LCProfilePicture>();
My first reaction to that is, since it is static, are you going to need it to be thread safe. If the answer is yes, maybe, or even no, then the answer might be, don't write it yourself, let Microsoft do it.
System.Collections.Concurrent.ConcurrentDictionary<TKey, TValue>
Which so happens to have 2 built in functions
TValue GetOrAdd(TKey key, TValue value)
TValue GetOrAdd(TKey key, Func<TKey, TValue> func)
And all of that done in a thread-safe manner.
The second one where the parameter is a Func is the maybe the answer you are looking for.
If you are set on simplifying the usage, I would argue against having the loading of the data be part of the TValue. That is mostly based on my own person preference to store POCO (Plain Old CLR Objects) as values is Dictionaries and not objects with State and Behavior.
I would instead, move the "loading/constructing/deserializing" behavior to another service and/or the Dictionary itself.
This example creates a base class that you inherit from
public abstract class SmartConcurrentDictionaryBase<TKey, TValue> :
System.Collections.Concurrent.ConcurrentDictionary<TKey, TValue>
{
public TValue GetOrAdd(TKey key) { return GetOrAdd(key, LoadNewValue); }
protected abstract TValue LoadNewValue(TKey key);
}
public class LCProfilePictureDictionary : SmartConcurrentDictionaryBase<int, LCProfilePicture>
{
protected override LCProfilePicture(int accountID)
{
return new LCProfilePicture(accountID);
}
}
// use is
// var pic = All.GetOrAdd(accountID);
This example is more of a reusable Dictionary object and takes in a Func as a constructor parameter, but could easily be changed to include an Interface where one of the functions on the interface match the pattern.
public class SimpleConcurrentDictionary<TKey, TValue> :
System.Collections.Concurrent.ConcurrentDictionary<TKey, TValue>
{
private readonly Func<TKey, TValue> _loadFunc;
public SimpleConcurrentDictionary(Func<TKey, TValue> loadFunc)
{
_loadFunc = loadFunc;
}
public TValue GetOrAdd(TKey key) { return GetOrAdd(key, _loadFunc); }
}
System.Reflection has a ConstructorInfo object and a GetConstructor method that can be used for this purpose. ConstructorInfo.Invoke returns an object of the type that you used to create your ConstructorInfo. If you went the reflection route, it would look something like this (not tested, but should be close):
//using System.Reflection;
public static TValue GetOrCreateEntry<TKey, TValue>(Dictionary<TKey, TValue> dict, TKey key)
{
TValue value;
if (!dict.TryGetValue(key, out value))
{
// not in dictionary
ConstructorInfo ctor = typeof(TValue).GetConstructor(new Type[] { typeof(TKey) });
if (ctor != null)
{
// we have a constructor that matches the type you need
value = (TValue)ctor.Invoke(new object[] { key });
dict[key] = value;
return value;
}
else
throw new NotImplementedException(); // because the TValue type does not implement the constructor you anticipate
}
// we got it from dictionary, so just return it
return value;
}
Solution #1 (most general):
public static TValue GetOrCreateEntry<TKey, TValue>( Dictionary<TKey, TValue> dict, TKey key, Func<TValue> creator )
{
TValue value;
if (!dict.TryGetValue( key, out value )) {
value = creator();
dict[ key ] = value;
}
return value;
}
Example usage:
static LCProfilePicture GetOrCreatePic( int accountID )
{
return GetOrCreateEntry<int, LCProfilePicture>( All, accountID, () => new LCProfilePicture( accountID ) );
}
Solution #2 (for TValues that remember their key):
public static TValue GetOrCreateEntry<TKey, TValue>( Dictionary<TKey, TValue> dict, TKey key, Func<TKey, TValue> creator )
{
TValue value;
if (!dict.TryGetValue( key, out value )) {
value = creator(key);
dict[ key ] = value;
}
return value;
}
Example usage:
static LCProfilePicture GetOrCreatePic( int accountID )
{
return GetOrCreateEntry<int, LCProfilePicture>( All, accountID, key => new LCProfilePicture( key ) );
}
Comparison of Solution #1 and Solution 2:
Solution #1 is more general - it can be used even for TValues that don't need to know about the key.
Solution #2 is cleaner style, for TValues that do retain a reference to the key.
Two reasons #2 is preferable, where appropriate:
Reason #1: Solution #1 has the possibility of abuse: consider the case where TValue has two constructors, a parameterless one, and one that takes key as a parameter. An inexperienced programmer might use Solution #1 like this:
static LCProfilePicture GetOrCreatePic( int accountID )
{
// OOPS, programmer has not set the key field to "accountID".
return GetOrCreateEntry<int, LCProfilePicture>( All, accountID, () => new LCProfilePicture() );
}
If the lead programmer / architect wants to avoid that possibility, omit Solution #1, and only offer #2. In that case, the attempted usage won't compile, because there is no matching constructor.
Reason #2: Using Solution #1 requires including a second copy of the key in the usage, if TValue needs to capture it. This unnecessarily encapsulates the key in the Func instance, and could lead to accidentally referring to a different key, e.g.:
//...
int keyA;
int keyB;
// OOPS, programmer referred to the wrong key the second time.
// Maybe copy/pasted code, and only changed it in the first location, not realizing it is used in two places.
var valA = GetOrCreateEntry<int, LCProfilePicture>( All, keyA, () => new LCProfilePicture( keyB) );
enter code here
I am sick of doing blocks of code like this for various bits of code I have:
if (dict.ContainsKey[key]) {
dict[key] = value;
}
else {
dict.Add(key,value);
}
and for lookups (i.e. key -> list of value)
if (lookup.ContainsKey[key]) {
lookup[key].Add(value);
}
else {
lookup.Add(new List<valuetype>);
lookup[key].Add(value);
}
Is there another collections lib or extension method I should use to do this in one line of code no matter what the key and value types are?
e.g.
dict.AddOrUpdate(key,value)
lookup.AddOrUpdate(key,value)
As Evgeny says, the indexer will already replace existing values - so if you just want to unconditionally set the value for a given key, you can do
dictionary[key] = value;
The more interesting case is the "get a value, or insert it if necessary". It's easy to do with an extension method:
public static TValue GetOrCreateValue<TKey, TValue>
(this IDictionary<TKey, TValue> dictionary,
TKey key,
TValue value)
{
return dictionary.GetOrCreateValue(key, () => value);
}
public static TValue GetOrCreateValue<TKey, TValue>
(this IDictionary<TKey, TValue> dictionary,
TKey key,
Func<TValue> valueProvider)
{
TValue ret;
if (!dictionary.TryGetValue(key, out ret))
{
ret = valueProvider();
dictionary[key] = ret;
}
return ret;
}
Note the use of a delegate to create the default value - that facilitates scenarios like the "list as value" one; you don't want to create the empty list unless you have to:
dict.GetOrCreateValue(key, () => new List<int>()).Add(item);
Also note how this only performs the lookup once if the key is already present - there's no need to do a ContainsKey and then look up the value. It still requires two lookups when it's creating the new value though.
When updating you don't need to perform a check. Simply use:
dict[key] = value
It will replace any existing value. When retrieving the value unfortunately there is no convenient single method (like setdefault in Python), but you could make your own extension method. Something like this:
if (!lookup.TryGetValue(key, out value))
{
value = new List<T>();
lookup.Add(key, value);
}
ConcurrentDictionary in .NET 4.0 has this nice method. You could also write an extension method for this.
If working with .NET Framework 4 or later, you can use the AddOrUpdate Method
dict.AddOrUpdate(key,value)
add or update is like this
dict[key] = value;
I like AddOrUpdate method of ConcurrentDictionary, but I like performance of Dictionary collection too :) So, this is extension method for all classes implementing IDictionary.
public static TValue AddOrUpdate<TKey, TValue>(
this IDictionary<TKey, TValue> dict,
TKey key,
TValue addValue,
Func<TKey, TValue, TValue> updateValueFactory)
{
TValue existing;
if (dict.TryGetValue(key, out existing))
{
addValue = updateValueFactory(key, existing);
dict[key] = addValue;
}
else
{
dict.Add(key, addValue);
}
return addValue;
}
public static TValue AddOrUpdate<TKey, TValue>(
this IDictionary<TKey, TValue> dict,
TKey key,
Func<TKey, TValue> addValueFactory,
Func<TKey, TValue, TValue> updateValueFactory)
{
TValue existing;
if (dict.TryGetValue(key, out existing))
{
existing = updateValueFactory(key, existing);
dict[key] = existing;
}
else
{
existing = addValueFactory(key);
dict.Add(key, existing);
}
return existing;
}
Im not sure if there is a method like you ask for, but you could write a small function for it, or use the try catch exception, presumably if you try adding a value that already exists it will throw an exception. If you catch that and ignore it... Just a suggestion
The indexer into Dictionary throws an exception if the key is missing. Is there an implementation of IDictionary that instead will return default(T)?
I know about the TryGetValue() method, but that's impossible to use with LINQ.
Would this efficiently do what I need?:
myDict.FirstOrDefault(a => a.Key == someKeyKalue);
I don't think it will as I think it will iterate the keys instead of using a Hash lookup.
Indeed, that won't be efficient at all.
As per comments, in .Net Core 2+ / NetStandard 2.1+ / Net 5, MS added the extension method GetValueOrDefault()
For earlier versions you can write the extension method yourself:
public static TValue GetValueOrDefault<TKey,TValue>
(this IDictionary<TKey, TValue> dictionary, TKey key)
{
TValue ret;
// Ignore return value
dictionary.TryGetValue(key, out ret);
return ret;
}
Or with C# 7.1:
public static TValue GetValueOrDefault<TKey,TValue>
(this IDictionary<TKey, TValue> dictionary, TKey key) =>
dictionary.TryGetValue(key, out var ret) ? ret : default;
That uses:
An expression-bodied method (C# 6)
An out variable (C# 7.0)
A default literal (C# 7.1)
If you're using .NET Core 2 or above (C# 7.x), the CollectionExtensions class is introduced and you can use the GetValueOrDefault method to get default value if key is not there in a dictionary.
Dictionary<string, string> colorData = new Dictionary<string, string>();
string color = colorData.GetValueOrDefault("colorId", string.Empty);
Carrying these extension methods can help..
public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key)
{
return dict.GetValueOrDefault(key, default(V));
}
public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key, V defVal)
{
return dict.GetValueOrDefault(key, () => defVal);
}
public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dict, K key, Func<V> defValSelector)
{
V value;
return dict.TryGetValue(key, out value) ? value : defValSelector();
}
Collections.Specialized.StringDictionary provides a non-exception result when looking up a missing key's value. It is also case-insensitive by default.
Caveats
It is only valid for its specialized uses, and — being designed before generics — it doesn't have a very good enumerator if you need to review the whole collection.
If you're using .Net Core, you can use the CollectionExtensions.GetValueOrDefault method. This is the same as the implementation provided in the accepted answer.
public static TValue GetValueOrDefault<TKey,TValue> (
this System.Collections.Generic.IReadOnlyDictionary<TKey,TValue> dictionary,
TKey key);
public class DefaultIndexerDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
private IDictionary<TKey, TValue> _dict = new Dictionary<TKey, TValue>();
public TValue this[TKey key]
{
get
{
TValue val;
if (!TryGetValue(key, out val))
return default(TValue);
return val;
}
set { _dict[key] = value; }
}
public ICollection<TKey> Keys => _dict.Keys;
public ICollection<TValue> Values => _dict.Values;
public int Count => _dict.Count;
public bool IsReadOnly => _dict.IsReadOnly;
public void Add(TKey key, TValue value)
{
_dict.Add(key, value);
}
public void Add(KeyValuePair<TKey, TValue> item)
{
_dict.Add(item);
}
public void Clear()
{
_dict.Clear();
}
public bool Contains(KeyValuePair<TKey, TValue> item)
{
return _dict.Contains(item);
}
public bool ContainsKey(TKey key)
{
return _dict.ContainsKey(key);
}
public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
{
_dict.CopyTo(array, arrayIndex);
}
public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
{
return _dict.GetEnumerator();
}
public bool Remove(TKey key)
{
return _dict.Remove(key);
}
public bool Remove(KeyValuePair<TKey, TValue> item)
{
return _dict.Remove(item);
}
public bool TryGetValue(TKey key, out TValue value)
{
return _dict.TryGetValue(key, out value);
}
IEnumerator IEnumerable.GetEnumerator()
{
return _dict.GetEnumerator();
}
}
One could define an interface for the key-lookup function of a dictionary. I'd probably define it as something like:
Interface IKeyLookup(Of Out TValue)
Function Contains(Key As Object)
Function GetValueIfExists(Key As Object) As TValue
Function GetValueIfExists(Key As Object, ByRef Succeeded As Boolean) As TValue
End Interface
Interface IKeyLookup(Of In TKey, Out TValue)
Inherits IKeyLookup(Of Out TValue)
Function Contains(Key As TKey)
Function GetValue(Key As TKey) As TValue
Function GetValueIfExists(Key As TKey) As TValue
Function GetValueIfExists(Key As TKey, ByRef Succeeded As Boolean) As TValue
End Interface
The version with non-generic keys would allow code that was using code using non-structure key types to allow for arbitrary key variance, which would not be possible with a generic type parameter. One should not be allowed to use a mutable Dictionary(Of Cat, String) as a mutable Dictionary(Of Animal, String) since the latter would allow SomeDictionaryOfCat.Add(FionaTheFish, "Fiona"). But there's nothing wrong with using a mutable Dictionary(Of Cat, String) as an immutable Dictionary(Of Animal, String), since SomeDictionaryOfCat.Contains(FionaTheFish) should be considered a perfectly well-formed expression (it should return false, without having to search the dictionary, for anything that isn't of type Cat).
Unfortunately, the only way one will be able to actually use such an interface is if one wraps a Dictionary object in a class which implements the interface. Depending upon what you're doing, however, such an interface and the variance it allows might make it worth the effort.
If you are using ASP.NET MVC, you could leverage the RouteValueDictionary class that do the job.
public object this[string key]
{
get
{
object obj;
this.TryGetValue(key, out obj);
return obj;
}
set
{
this._dictionary[key] = value;
}
}
I used encapsulation to create an IDictionary with behavior very similar to an STL map, for those of you who are familiar with c++. For those who aren't:
indexer get {} in SafeDictionary below returns the default value if a key is not present, and adds that key to the dictionary with a default value. This is often the desired behavior, as you're looking up items that will appear eventually or have a good chance of appearing.
method Add(TK key, TV val) behaves as an AddOrUpdate method, replacing the value present if it exists instead of throwing. I don't see why m$ doesn't have an AddOrUpdate method and thinks throwing errors in very common scenarios is a good idea.
TL/DR - SafeDictionary is written so as to never throw exceptions under any circumstances, other than perverse scenarios, such as the computer being out of memory (or on fire). It does this by replacing Add with AddOrUpdate behavior and returning default instead of throwing NotFoundException from the indexer.
Here's the code:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
public class SafeDictionary<TK, TD>: IDictionary<TK, TD> {
Dictionary<TK, TD> _underlying = new Dictionary<TK, TD>();
public ICollection<TK> Keys => _underlying.Keys;
public ICollection<TD> Values => _underlying.Values;
public int Count => _underlying.Count;
public bool IsReadOnly => false;
public TD this[TK index] {
get {
TD data;
if (_underlying.TryGetValue(index, out data)) {
return data;
}
_underlying[index] = default(TD);
return default(TD);
}
set {
_underlying[index] = value;
}
}
public void CopyTo(KeyValuePair<TK, TD>[] array, int arrayIndex) {
Array.Copy(_underlying.ToArray(), 0, array, arrayIndex,
Math.Min(array.Length - arrayIndex, _underlying.Count));
}
public void Add(TK key, TD value) {
_underlying[key] = value;
}
public void Add(KeyValuePair<TK, TD> item) {
_underlying[item.Key] = item.Value;
}
public void Clear() {
_underlying.Clear();
}
public bool Contains(KeyValuePair<TK, TD> item) {
return _underlying.Contains(item);
}
public bool ContainsKey(TK key) {
return _underlying.ContainsKey(key);
}
public IEnumerator<KeyValuePair<TK, TD>> GetEnumerator() {
return _underlying.GetEnumerator();
}
public bool Remove(TK key) {
return _underlying.Remove(key);
}
public bool Remove(KeyValuePair<TK, TD> item) {
return _underlying.Remove(item.Key);
}
public bool TryGetValue(TK key, out TD value) {
return _underlying.TryGetValue(key, out value);
}
IEnumerator IEnumerable.GetEnumerator() {
return _underlying.GetEnumerator();
}
}
It could be a one-liner to check TryGetValue and return default value if it is false.
Dictionary<string, int> myDic = new Dictionary<string, int>() { { "One", 1 }, { "Four", 4} };
string myKey = "One"
int value = myDic.TryGetValue(myKey, out value) ? value : 100;
myKey = "One" => value = 1
myKey = "two" => value = 100
myKey = "Four" => value = 4
Try it online
Since .NET core 2.0 you can use:
myDict.GetValueOrDefault(someKeyKalue)
What about this one-liner that checks whether a key is present using ContainsKey and then returns either the normally retreived value or the default value using the conditional operator?
var myValue = myDictionary.ContainsKey(myKey) ? myDictionary[myKey] : myDefaultValue;
No need to implement a new Dictionary class that supports default values, simply replace your lookup statements with the short line above.
This question helped to confirm that the TryGetValue plays the FirstOrDefault role here.
One interesting C# 7 feature I would like to mention is the out variables feature, and if you add the null-conditional operator from C# 6 to the equation your code could be much more simple with no need of extra extension methods.
var dic = new Dictionary<string, MyClass>();
dic.TryGetValue("Test", out var item);
item?.DoSomething();
The downside of this is that you can't do everything inline like this;
dic.TryGetValue("Test", out var item)?.DoSomething();
If we'd need/want to do this we should code one extension method like Jon's.
Here is a version of #JonSkeet's for the world of C# 7.1 that also allows for an optional default to be passed in:
public static TV GetValueOrDefault<TK, TV>(this IDictionary<TK, TV> dict, TK key, TV defaultValue = default) => dict.TryGetValue(key, out TV value) ? value : defaultValue;
It may be more efficient to have two functions to optimize the case where you want to return default(TV):
public static TV GetValueOrDefault<TK, TV>(this IDictionary<TK, TV> dict, TK key, TV defaultValue) => dict.TryGetValue(key, out TV value) ? value : defaultValue;
public static TV GetValueOrDefault2<TK, TV>(this IDictionary<TK, TV> dict, TK key) {
dict.TryGetValue(key, out TV value);
return value;
}
Unfortunately C# doesn't (yet?) have a comma operator (or the C# 6 proposed semicolon operator) so you have to have an actual function body (gasp!) for one of the overloads.
Modern Answer
As of .NET Core 2.0, there is a built-in extension method with 2 overloads:
TValue GetValueOrDefault<TKey,TValue>(TKey)
TValue GetValueOrDefault<TKey,TValue>(TKey, TValue)
Usage:
var dict = new Dictionary<string, int>();
dict.GetValueOrDefault("foo"); // 0: the datatype's default
dict.GetValueOrDefault("foo", 2); // 2: the specified default
The first version returns null for nullable types, of course.
See the documentation for more detail.
In general I would support the answer from Jon Skeet, however I prefer an implementation where I can give the default value as parameter:
public static TValue GetValueOrDefault<TKey, TValue> (this IDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
{
if (dictionary.ContainsKey(key))
return dictionary[key];
else
return defaultValue;
}
No, because otherwise how would you know the difference when the key exists but stored a null value? That could be significant.